1. Field of the Invention
The invention relates generally to security features in printed documents and more specifically to visible authentication patterns in printed documents. The visible authentication patterns can be used to distinguish original printed documents from photocopies of those printed documents, to detect alterations in documents, and to carry hidden and/or visible messages.
2. Description of Related Art
A prerequisite for a commercial society is being able to distinguish authentic items from false or counterfeit items. With documents, the questions that may need to be answered to determine a document's authenticity include the following:                Is the document an original or a copy of an original?        Has the document been altered since it was first made?        Is the document authorized?        
Many techniques have been developed to make it reasonably possible to answer these questions from the document itself. Techniques which make it easier to determine whether a document is an original or a copy include the complex etchings used in paper money, the photographs used in ID cards, and special papers. Special papers and inks have been used to detect alterations. Techniques for showing that a document is authorized have included signatures and seals.
With the advent of digital scanning and printing techniques, digital watermarks have been used to authenticate printed documents. A digital watermark is a message that is embedded in a digital representation of a document by adding noise that carries the message to a graphic element in the document. When used for authentication purposes, the message is generally invisible and can be read only if the location of the information making up the message in the graphic element is known. For a survey of digital watermarking techniques, see Appendix A, beginning at col. 17, line 27 of U.S. Pat. No. 6,345,104, Rhoads, Digital watermarks and methods for security documents, issued Feb. 5, 2002, which is incorporated herein by reference for all purposes. For an example of how digital watermarks may be used for authentication purposes, see U.S. Pat. No. 6,243,480, Jian Zhao, Digital authentication with analog documents, issued Jun. 5, 2001.
Progress in the technology of copying has diminished the value of all of the techniques that permit one to determine from the appearance of a document whether it is authentic. Because of this progress, counterfeiting not only of money and financial instruments, but also of other documents such as ID cards and diplomas as well as of packaging and labels causes huge losses, ranging to 5% to 8% of worldwide sales of branded products, and endangers the reputation and value of the brands themselves. Moreover, the growth of the Internet drives the business of counterfeited documents (fake IDs, university diplomas, checks, and so on), which can be bought easily and anonymously from hundreds of companies on the Web. As the precision of scanners, digital imaging software and printers increase, the problem will only get worse.
What is needed as the scanners, digital imaging software, and printers get better is new ways of adding information to a document to make it possible to determine whether it is an original or a copy, whether it has been altered, and/or whether it has been authorized. Efforts in this area have included the following:
Embedding Multiple Watermarks in a Document
U.S. Pat. No. 6,332,031, Rhoads, et al., Multiple watermarking techniques for documents and other data, issued Dec. 18, 2001, discloses embedding several watermarks in the document, each of them with different properties or in different domains. If the document is photocopied or scanned and printed in order to produce a counterfeit, the embedded watermarks will be altered or damaged. The properties of the watermark or the domain in which it is embedded will affect the degree to which it is altered by the copying process. Thus, the relative degree of alteration of each watermark can indicate whether the document is an original or copy. The use of watermarks in this fashion has a number of advantages:                It is flexible: a digital watermark can in theory be inserted into any document, because it merely introduces unnoticeable modifications to the document.        Because it is invisible, it can be used to determine the source of the counterfeits.        The possible presence of watermarks forces the counterfeiter to reproduce the whole document with very high fidelity.        
The advantages of watermarks are also their disadvantages. Because digital watermarks used for security purposes are made by adding invisible noise to a document, they often cannot be read where the document that contains them has been subject to wear and tear. Because they are hidden in the noise in a document, it is difficult to hide the watermarks in documents such as banknotes where every element of the design is fixed and there is thus no room for noise.
Embedding Information that Cannot be Reproduced by a Photocopier
A document may contain a part which is invisible in the visible light range because it is printed with an ink that is visible in ultra-violet light. The photocopier, which operates using visible light, cannot reproduce it. See U.S. Pat. No. 5,868,432, Mantegazza, Documents with anticopying means to prevent reproducibility by photocopying, issued Feb. 9, 1999.
The information may still require more resolution than the scanning-printing process is capable of. See U.S. Pat. No. 5,708,717, Alosia, Digital anti-counterfeiting software method and apparatus, issued Nov. 13, 1998, which discloses a method for combining a source image and a latent image which is visible only when viewed through a special decoder lens. This latent image can, for example contain the words “authentic” repeated several time, or more document-specific information such as the personal information in the portrait of a ID card. However, since the latent image is printed with “sub-pixel” precision, it cannot be easily reproduced. Of course, what is “sub-pixel” today may be easily reproducible tomorrow.
Holograms are inserted on documents such as ID cards and banknotes because they are assumed to be easy to detect by the human eye and hard to reproduce with high fidelity. However, while anyone can see whether a hologram is present on a document, an untrained observer will generally be unable to detect whether the hologram is authentic or a copy.
Common Problems of Invisible Copy Protection Features
A problem that all of the invisible copy protection features have is that their invisibility makes them completely useless to people who do not have the special instruments needed to read them. Moreover, the invisibility of the features causes problems with printing and/or detection. With the watermarks, the need to keep the watermarks invisible necessarily makes them hard to detect, and that is particularly the case when wear and tear add extra noise to a document. With invisible ink, both printing and detection are complicated, and that is also the case with latent images printed with “sub-pixel precision”.
What is needed is techniques that can reliably determine, at a lower cost, whether a document is an original or a copy, whether the document has been altered, or whether it is authorized, that make it possible for the public to see that a document may be easily authenticated, and that can be easily integrated with other techniques for authenticating a document. It is an object of the inventions disclosed herein to provide such techniques.